1. Field of the Invention
This invention relates to a nano-composite steel plate with layered nanostructure, which characterized by periodic distribution of nano-/micrograined layer resulting in a high strength and large ductility, and to a method of making such a steel plate.
2. Background of the Invention
The machinery industry requires steel materials that have high strength, enhanced formability and environment-friendly performance. The strength of steel materials is generally improved by alloying, i.e. by the addition of an alloy element, such as Cr, Ni, Mo, Mn, Ti and W and the like. However, alloying is relatively expensive and environmentally unfriendly. Another method of strengthening steel is grain refinement that introduces refined microstructures into the steel by thermomechanical and plastic deformation processes. These refined microstructures enhance the strength of the steel compared with the conventional coarse grain steel. The technique of strengthening by refinement attracts more and more attentions for its low cost, recycleability, high purification and little alloying.
The patent literature described several methods of introduces refined microstructures into the steel. Chinese patent 1297062 and European patent 1031632 describe a thermomechanical process for producing steel with a refined ferrite grain size of 3 μm or less, which involves heating the base plate at Ac3 point for austenizing, forging at a temperature range of between Ac3-150° C., or less than 550° C., at strain rate of 0.001-10/s, and then cooling to room temperature to obtain refined grains. Japanese patent 2000073 152 introduces an accumulative roll-bonding method by repeated stacking and rolling to refine the grain size to submicron scale. Chinese patent 127554 employs an integrated processing of pre-treatment by transformation, plastic deformation and recrystalline to achieve nanocrystalline plate with low alloying (CrM0V). However, the research shows that ultra-fine grained steels, that is steel having a grain size less than 1 μm, exhibit increased strength increases but lower plasticity. The deterioration of plasticity is accelerated with the decrease of the grain size. When the grain size extends to the nano-scale, the steels even exhibit a transition from ductile material to brittle material, which is very unfavorable for engineering applications.
In situ formed composite like microstructures, such as a bimodal grain size distribution, can attain large ductility induced by dislocation accumulation of coarse grains while maintaining the majority of the strengthening brought forth by nanostructure. This idea obtains a primary effect in pure Cu (Nature, 2002, 419:912) and Al alloys (Scripta Materialia, 2003, 49:297). An example can be found in Chinese patent publication 1655376A which describes the processing of submicron grained steel plate with nano-precipitates. However, these methods are limited to laboratory applications and are difficult to implement in industrial or commercial applications. Further, the properties of the resulting materials are instability for the inhomogeneous microstructure.
Chinese patent publications 1410560A and 1410560A, Chinese utility model 2604443Y, US patent publication 2003/0127160 A1 and Japanese patent 2003183730, describe various surface nanocrystalline techniques aimed at overcoming the above disadvantages. The common characters of these kinds of surface nanocrystalline techniques are refinement of grain size to nano scale in steel surface in certain depth by using mechanical processing, or transformation treatment. A good combination of mechanical properties is developed by utilizing the fine grain strengthening in the steel surface layer and plasticity providing by conventional grain in centre. However, an obvious disadvantage of these treatments is limited in strengthening much lower than the ultra-fine grained materials (d<1 μm), since the thickness of the nanocrystallized layer is generally within a depth of 50 μm resulting in a volume fraction of nanocrystalline lower than 5% even in a sheet steel. For example, the strength of low carbon steel with a thickness of 3% nanostructured layers is enhanced by 35% treated by surface mechanical attrition treatment (Scripta Mater, 2001, 44(8/9):1791). As to a 316L stainless steel with 2% volume fraction of nanostructured layer, the tensile strength is increased 13% (Mater. Sci. Eng. A, 2004, 375-377:38). Hence, the influence of surface nanocrystalline on the strength is reduced gradually when the thickness of the steel plate increases. Therefore, the current techniques can not meet the demands of the high strength, large ductility and toughness of nano materials.